Polarization component, polarization light source and image display apparatus using the same

ABSTRACT

A polarization component, capable of efficiently reflecting an obliquely transmitted light beam toward a light source without degrading the transmission-polarization property of a perpendicular incident light beam, is provided. A C-plate having an oblique retardation of at least λ/8 with respect to a light beam inclined by at least 30° is disposed between at least two reflective circular polarizer layers whose selective reflection wavelength bands of polarized light overlapping each other. A combination of a reflective linear polarizer and a quarter wavelength plate may be used instead of the reflective circular polarizer. Alternatively, a combination of two reflective linear polarizer layers and two quarter wavelength plate layers (Nz≧2) disposed therebetween can provide a similar effect. Further, a combination of two reflective linear polarizer layers and a half wavelength plate (Nz≧1.5) disposed therebetween may be used. When reflective linear polarizer layers are used, they must be bonded together with their axial directions set at a certain angle. The polarization component is preferably used in various image display apparatuses such as liquid crystal display apparatuses and organic EL display apparatuses.

This application is a division of U.S. application Ser. No. 12/236,976filed Sep. 24, 2008, which is a division of U.S. Pat. No. 7,443,585filed Oct. 28, 2008, which is a U.S. national stage of PCT/JP03/04872filed Apr. 17, 2003, and which hereby incorporated by reference in itsentirety.

TECHNICAL FIELD

The present invention relates to a polarization component. Morespecifically, the present invention relates to a polarization componentthat has excellent efficiency in use of diffuse light emitted from alight source, and that is used suitably for a high-brightnesspolarization light source and various image display apparatuses such asa highly-visible liquid crystal display apparatus, an organic EL displayapparatus, PDP, and CRT.

BACKGROUND ART

For the purpose of improving visibility or the like of an image displayapparatus, a technique of focusing light beams emitted from a lightsource toward a front direction so as to enhance the brightness has beenused in general. More specifically, for example, a lens, a mirror(reflective layer) a prism or the like is used for utilizing refractionand reflection for focusing and parallelizing light beams, therebyenhancing the brightness.

For example, in a liquid crystal display apparatus, a light beam emittedfrom a light source is focused in a front direction by a prism sheet orthe like so as to make the light beam enter a liquid crystal displayapparatus efficiently to enhance the brightness. However, since a largerefractive index difference is required in principle when focusing bymeans of a prism sheet, the prism sheet must be disposed via an airlayer or the like. This may result in an optical loss caused byunnecessary reflection or scattering. Another problem is that a largenumber of parts will be required.

For another technique of enhancing emission brightness in polarization,a brightness enhancement system that uses retro-reflection has beenproposed. Specifically, this brightness enhancement system includesdisposing a reflective layer on a bottom surface of a light-guidingplate and disposing a reflective polarizer on the light-emittingsurface. A light beam entering the system is separated into transmittedlight and reflected light depending on its polarization state, and thereflected light is reflected via a reflective layer on the bottomsurface of the light-guiding plate and re-emitted from the emissionsurface so as to enhance the brightness. For example, reflection andseparation of circularly polarized light by a cholesteric liquid crystalis detailed in, for example, JP 03 (1991)-45906 A, JP6 (1994)-324333 A,and JP 07 (1995)-36032 A. However, such a brightness enhancement systemcannot provide sufficient effects with respect to a light source whosefocusing property is improved previously by using a prism sheet or thelike, in comparison to a case where it is applied to a light sourcehaving a strong diffusivity.

For solving the above-described problems, techniques for enhancingbrightness have been studied for parallelizing light beams from a lightsource by using a special optical film instead of a lens, a mirror, aprism or the like. A representative example is a method of using acombination of a line spectrum light source and a bandpass filter. Morespecific examples includes a method of disposing a bandpass filter on aline-luminescent light source such as a CRT or electroluminescence or adisplay apparatus, as described in applications or issued patents ofPhilips, for example, JP 06 (1994)-235900 A, JP 02 (1990)-158289 A,Tokuhyo 10 (1998)-510671 A (published Japanese translation of PCTinternational publication for patent application), U.S. Pat. No.6,307,604, DE3836955, DE4220289, EP578302, US2002-0034009, WO002/25687,or US2001-521643 and US 2001-516066. Another example of a technique asdescribed in US2002-0036735 (Fuji Photo Film Co., Ltd.) includesdisposing a bandpass filter corresponding to three wavelengths, withrespect to a line spectrum type cold cathode ray tube. However, thesetechniques have disadvantages that they will not function to the lightsources without a line spectrum, or they have problems in designing andmanufacturing films that selectively function with respect to a specificwavelength. Furthermore, an evaporated interference film is often usedfor the bandpass filter, but it has disadvantages, for example, that thewavelength properties may change under a humidified atmosphere, due to achange in the refractive index of the thin films.

Examples of a light-parallelizing system that uses a hologram-basedmaterial include a system described in U.S. Pat. No. 4,984,872 A1(Rockwell International Corporation). However, the material has a highfront transmittance while its reflection-elimination rate for obliqueincident light beams is not so high. When a parallel light beam isprovided to this system for a calculation of the straight transmittance,the transmittance in the front direction will be measured high since thelight passes through in the front direction, while an oblique incidentlight beam will be scattered so that measurement value for thetransmittance will be low. However, the difference will not occur on adiffusion light source. Therefore, for a case of disposing the system ona diffusion backlight light source in use, it cannot exhibit necessarilyits focusing function sufficiently. Moreover, the hologram-basedmaterial has problems in its physical properties such as durability,reliability or the like.

DISCLOSURE OF INVENTION

Therefore, it is an object of the present invention to provide apolarization component that can reflect obliquely-transmitted lighttoward a light source efficiently without degrading atransmission-polarization property of perpendicular incident light.

For solving the above-mentioned problems, a polarization component ofthe present invention includes, at least, two reflective polarizerlayers and a retardation layer disposed between the reflective polarizerlayers, where the two reflection polarizer layers are reflectivecircular polarizer layers that selectively transmit one of clockwisecircularly polarized light or counterclockwise circularly polarizedlight while selectively reflect the other, and wherein the tworeflective circular polarizer layers have selective reflectionwavelength bands for selective reflection of polarized light, the bandsoverlapping each other at least partially, and the retardation layersatisfies conditions of the Formulae (I) and (II) below.

R<(λ/10)  (I)

R′>(λ/8)  (II)

wherein in Formulae (I) and (II), A denotes a wavelength of lightentering the retardation layer;

R denotes an absolute value of retardation (in-plane retardation)between a X-axis direction and a Y-axis direction with respect toincident light from a Z-axis direction (normal direction), where theX-axis direction is a direction showing a maximum refractive indexwithin the plane of the retardation layer (in-plane slow axisdirection), the Y-axis direction is a direction perpendicular to theX-axis direction within the plane of the retardation layer (in-planefast axis direction), and the Z-axis direction is a thickness directionof the retardation layer and perpendicular to the X-axis direction andthe Y-axis direction;

R′ denotes an absolute value of retardation between a X′-axis directionand a Y′-axis direction with respect to incident light from a directioninclined by at least 30° with respect to the Z-axis direction, where theX′-axis direction is an axial direction within a plane of theretardation layer perpendicular to the incidence direction of theincident light inclined by at least 30° with respect to the Z-axisdirection, and the Y′-axis direction is a direction perpendicular to theincidence direction and to the X′-axis direction.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a mechanism of simultaneous expression of focusing andbrightness enhancement in one embodiment of a polarization component ofthe present invention, where a reflective circular polarizer and aC-plate are combined.

FIG. 2 is an explanatory view to show signs that represent naturallight, circularly polarized light and linearly polarized light,according to the present Invention.

FIG. 3 is a schematic diagram of circular polarization by a combinationof a linear polarizer and a quarter wavelength plate.

FIG. 4 shows a mechanism of simultaneous expression of focusing andbrightness enhancement in one embodiment of a polarization component ofthe present invention, where a reflective linear polarizer, a C-plateand a quarter wavelength plate are combined.

FIG. 5 is a schematic view to show angles formed by respective layers inthe polarization component of FIG. 4.

FIG. 6 shows a mechanism of simultaneous expression of focusing andbrightness enhancement in one embodiment of a polarization component ofthe present invention, where a reflective linear polarizer and a quarterwavelength plate (Nz≧2) are combined.

FIG. 7 is a schematic view to show angles formed by respective layers inthe polarization component of FIG. 6.

FIG. 8 shows a mechanism of simultaneous expression of focusing andbrightness enhancement in one embodiment of a polarization component ofthe present invention, where a reflective linear polarizer and a halfwavelength plate (Nz≧1.5) are combined.

FIG. 9 is a schematic view to show angles formed by respective layers inthe polarization component of FIG. 8.

FIG. 10 is a schematic view to show one example of optical properties ofa negative C-plate.

FIG. 11 is a schematic view of a retardation layer includinghomeotropically-aligned liquid crystal molecules.

FIG. 12 is a schematic view of a retardation layer including a discoticliquid crystal.

FIG. 13 is a schematic view of a retardation layer including aninorganic layered compound.

FIG. 14 shows one example of bonding angles between respective layers ina case of combining a reflective linear polarizer, a C-plate and aquarter wavelength plate, according to a polarization component of thepresent invention.

FIG. 15 is an explanatory view to show a conversion path for light beamsin the polarization component of FIG. 14, indicated with a Poincaresphere.

FIG. 16 is a graph to show performance in focusing and brightnessenhancement for a polarization component of Example 1.

FIG. 17 is a graph to show performance in focusing and brightnessenhancement for polarization components of Examples 5 and 6.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, embodiments of the present invention will be described below.

As a result of keen studies, the inventors have found that apolarization component of the present invention, having theabove-described configuration, realizes efficient reflection ofobliquely transmitted light toward a light source without degrading thetransmission-polarization property of perpendicular incident light thatcontributes to front brightness. It is also possible to further enhancethe brightness by converting the obliquely-transmitted light (reflectivepolarization) reflected toward the light source into a light beam thatcan contribute to enhancement of the front brightness. Furthermore,since a polarization component of the present invention is provided withbrightness-enhancing function due to focusing and retro-reflection, itis less dependent on types of the light source regarding the focusingfunction and light-parallelizing function.

In a retardation layer of the present invention, the in-planeretardation R is not more than (λ/10) as described above, preferably itshould be reduced as much as possible from aspects of retaining thepolarization state of the incident light from the Z-axis direction(normal direction), preferably, λ/20 or less, more preferably, λ/50 orless, and ideally zero. Such a retardation layer that has no orextremely small in-plane retardation and has retardation only in thethickness direction is called a C-plate in which an optical axis ispresent in a thickness direction perpendicular to the in-planedirection. The C-plate is called a positive C-plate when its opticalproperty condition satisfies the following Formula (VI), and it iscalled a negative C-plate when its optical property condition satisfiesthe following Formula (VII). Examples of typical negative C-platesinclude films of biaxially-stretched polycarbonate (PC) and polyethyleneterephthalate (PET), a film of a cholesteric liquid crystal having aselective reflection wavelength band set to be shorter than that ofvisible light, a film having a discotic liquid crystal aligned inparallel with respect to the plane, and a product that can be obtainedby in-plane alignment of an inorganic crystal compound having a negativeretardation. Examples of typical positive C-plates include aperpendicularly-aligned liquid crystal film.

nx≈ny<nz  (VI)

nx≈ny>nz  (VII)

In the present invention, nx, ny and nz denote refractive indices indirections of a X-axis, a Y-axis, and a Z-axis in every optical layersuch as the above-mentioned C-plate. The X-axis direction is a directionshowing a maximum refractive index within the plane of the layer(in-plane slow axis direction), the Y-axis direction is a directionperpendicular to the X-axis direction within the plane of the layer(in-plane fast axis direction), and the Z-axis direction is a thicknessdirection of the layer and perpendicular to the X-axis direction and tothe Y-axis direction.

The retardation layer in the present invention is not limitedparticularly as long as it satisfies the optical property conditions ofFormulae (I) and (II). Preferably for example, the retardation layerincludes a cholesteric liquid crystal compound fixed in a planaralignment state, and has a selective reflection wavelength band presentin a wavelength region other than a visible light region (380 nm to 780nm). Here, the selective reflection wavelength band is set to be awavelength region other than a visible light region (380 nm to 780 nm)in order to avoid coloration or the like in the visible light region.The selective reflection wavelength band of the cholesteric liquidcrystal layer can be determined unequivocally on the basis of acholesteric chiral pitch and a refractive index of the liquid crystal,and a central wavelength λ of the selective reflection can berepresented by the following Formula (VIII).

λ=np  (VIII)

In Formula (VIII), n denotes an average refractive index of cholestericliquid crystal molecules, and p denotes a chiral pitch.

The value of the central wavelength of the selective reflectionwavelength band can be present in a region of a wavelength longer thanthat of visible light, e.g., it can be present in a near infraredradiation region. However, it is more preferable that the value ispresent in a ultraviolet region of not more than 350 nm, since anycomplicated phenomenon may not occur there substantially underinfluences or the like of optical rotation.

Though the kind of the cholesteric liquid crystal is not limitedparticularly and can be selected suitably, examples thereof include apolymeric liquid crystal obtained by polymerizing liquid crystalmonomers, a liquid crystal polymer exhibiting a cholesteric liquidcrystal property at a high temperature, and a mixture thereof. Thoughthe cholesteric liquid crystal property can be either lyotropic orthermotropic, a thermotropic liquid crystal is preferred from aspects ofeasy control and easy formation of monodomain. Similarly, the method ofproducing the cholesteric liquid crystal is not limited particularly,but any known methods can be used suitably. Materials that can be usedfor producing a partially-crosslinked polymer material having acholesteric liquid crystal property are not limited particularly.Arbitrary examples include materials as described in Tokuhyo 2002-533742(WO00/37585), EP358208 (U.S. Pat. No. 5,211,877), and EP66137 (U.S. Pat.No. 4,388,453). The cholesteric liquid crystal can be obtained also by,for example, mixing and reacting a nematic liquid crystal monomer or apolymerizable mesogenic compound with a chiral agent. The polymerizablemesogenic compound is not particularly limited but the examples can bethose disclosed in WO93/22397, EP0261712, DE19504224, DE4408171 andGB2280445. Non-chiral compounds or chiral compounds can be used, and thecompounds can be mono-, di-, or multi-reactive, which can be synthesizedin a known manner. Specific examples of the polymerizable mesogeniccompounds include trade name LC242 (produced by BASF AG), trade name E7(produced by Merck Ltd.) and trade name LC-Sillicon-CC3767 (produced byWacker-Chemie GmbH). The chiral agent is not particularly limitedeither, but can be synthesized by a method described in WO 98/00428, forexample. More specifically, non-polymerizable chiral compounds such astrade name S101, trade name R811, trade name CB15 (produced by MerckLtd.) or a chiral agent such as trade name LC756 (produced by BASF AG)can be used.

The method for producing the retardation layer containing thecholesteric liquid crystal compound is not particularly limited, but aconventionally known method for producing a cholesteric liquid crystallayer can be used suitably. An example thereof includes coating acholesteric liquid crystal compound on a base having an alignment filmon the surface or a base having by itself a liquid crystal alignmentcapability, aligning the compound and fixing the alignment state.

The base can be, for example, an alignment layer obtained by forming afilm of polyimide, polyvinyl alcohol, polyester, polyarylate, polyamideimide, polyetherimide or the like on a base having a birefringenceretardation as small as possible such as triacetylcellulose or amorphouspolyolefin, and rubbing the surface of this film with a rayon cloth orthe like, or an alignment layer obtained by forming an obliquelydeposited layer of SiO₂ on a similar base. Other examples include a baseprovided with a liquid crystal alignment capability by stretching apolyethylene terephthalate (PET) film or a polyethylene naphthalate(PEN) film, a base provided with a fine roughness having a finealignment-controlling force by further treating the surface of thestretched film with abrasives such as colcothar or a rubbing cloth, or abase obtained by forming an alignment layer that generates an alignmentcontrolling force by light irradiation, such as azobenzene compounds, onthe stretched film.

A specific method of forming a retardation layer containing acholesteric liquid crystal compound on the base is as follows. First, asolution of a liquid crystal polymer is coated on a surface of the basehaving a liquid crystal alignment capability, and dried for forming aliquid crystal layer. There is no particular limitation about thesolvent, and examples thereof includes chlorinated solvents such asmethylene chloride, trichloroethylene and tetrachloroethane,ketone-based solvents such as acetone, methyl ethyl ketone (MEK) andcyclohexanone, aromatic solvents such as toluene, cyclic alkanes such ascycloheptane, amide-based solvents such as N-methylpyrrolidone, andether-based solvents such as tetrahydrofuran. They may be used alone orin combination. Moreover, the coating method is not limited either butcan be a conventionally known method such as spin coating, rollercoating, flow coating, printing, dip coating, film flow-expanding, barcoating or gravure printing. In an alternatively applicable method, aheat-molten product of a liquid crystal polymer, preferably aheat-molten product in a state exhibiting an isotropic phase is coatedin a similar manner in place of the above-mentioned solution, which isthen expanded to form a thinner layer and fixed while keeping the melttemperature as required. Such a method is advantageous in good hygienein the working condition, as no solvent is used.

Then, the alignment state of the cholesteric liquid crystal molecules inthe liquid crystal layer is fixed to obtain a desired retardation layer.The fixing method is not limited particularly, but any suitable methodscan be selected corresponding to the objectives of the invention. Forexample, the liquid crystal layer is heated to a temperature of at leasta glass transition point and lower than an isotropic phase transitionpoint so as to cause a planar alignment of the liquid crystal polymermolecules, and then cooled down to the temperature lower than the glasstransition point so as to form a glassy state, thereby fixing thealignment. Alternatively, the alignment also may be fixed by anirradiation of energy such as ultraviolet rays or ion beams in a stagewhere the alignment state is formed. In the above process, it is alsopossible to use a liquid crystal monomer in place of or together withthe liquid crystal polymer, which is aligned and polymerized by ionizingradiation such as an electron beam and ultraviolet ray or heat so as toprovide a polymeric liquid crystal. At this time, it is possible to adda chiral agent or an alignment auxiliary as required.

The base, when having a small birefringence for example, can beintegrated with a retardation layer containing the cholesteric liquidcrystal compound in use for a polarization component. In a case, forexample, where the thickness or the birefringence of the base mayinhibit the functions of the polarization component, the retardationlayer can be peeled off from the base or can be transferred to anotherbase in use.

For the retardation layer, a retardation layer containing a rodlikeliquid crystal compound fixed in a homeotropic alignment state is alsopreferred. Though the kind of the homeotropic liquid crystal is notlimited particularly and can be selected suitably, the examples includea polymeric liquid crystal formed by polymerizing a liquid crystalmonomer, a liquid crystal polymer exhibiting a nematic liquid crystalproperty at a high temperature, and a mixture thereof. The polymericliquid crystal can be obtained by adding an alignment auxiliary or thelike to a liquid crystal monomer as required, and by polymerizing withionizing radiation such as an electron beam and ultraviolet ray or withheat. Though the liquid crystal property can be either lyotropic orthermotropic, a thermotropic liquid crystal is preferred from aspects ofeasy control and easy formation of monodomain. Though the liquid crystalmonomer is not limited particularly, the examples include apolymerizable mesogenic compound. Though the polymerizable mesogeniccompound is not limited particularly, the examples are similar to thecholesteric liquid crystals.

A method of forming such a retardation layer is not limitedparticularly, but a known method can be used suitably. For example, analignment film or the like can be used for the formation as in a case ofthe cholesteric liquid crystal. The homeotropic alignment can beobtained, for example, by coating the homeotropic liquid crystal on aperpendicularly-aligned film (e.g., long-chain alkylsilane) formedpreviously, expressing and fixing the liquid crystal state.

Furthermore, a retardation layer containing a discotic liquid crystalcompound fixed in a nematic phase or columnar phase alignment state ispreferred as well. Such a retardation layer can be formed, for example,by expressing a nematic phase or a columnar phase in a discotic liquidcrystal material having a negative uniaxiality (e.g., a compound ofphthalocyanine or triphenylene having a molecular spread within aplane), and fixing the state. Specific formation method is not limitedparticularly, and known methods can be used suitably.

Furthermore, the retardation layer preferably contains an inorganiclayered compound having a negative uniaxiality, and the inorganiclayered compound is in an alignment state where an optical axisdirection of the retardation layer is fixed to be perpendicular (normaldirection) to the plane. A method of forming the retardation layer isnot limited particularly, and known methods can be used suitably. Thenegative uniaxial inorganic layered compound is explained in detail inJP 06 (1994)-82777 A.

The schematic views of FIGS. 11-13 show respectively a retardation layerwith a fixed homeotropic alignment state, a retardation layer using adiscotic liquid crystal, and a retardation layer composed of aninorganic layered compound. In the drawings, reference numerals 1101,1201 and 1301 denote respectively a homeotropic liquid crystal molecule,a discotic liquid crystal molecule, and a flake of a negative-uniaxialinorganic layered compound crystal.

Furthermore, a retardation layer containing a biaxially-alignednon-liquid crystal polymer is also preferred. A method of forming theretardation layer is not limited particularly, but known methods can beused suitably. The examples include a method of biaxially stretching ina good balance a polymer film having a positive refractive anisotropy, amethod of pressing a thermoplastic resin, and a method of exciding aparallel-aligned crystalline. Alternatively, a solution of a certainnon-liquid crystal polymer is applied onto a base and dried, and formedas a film, thereby a C-plate can be obtained. Though the non-liquidcrystal polymer is not limited particularly, preferred examples include:polyester-based polymers such as polyethylene terephthalate andpolyethylene naphthalate; cellulose-based polymers such as diacetylcellulose and triacetyl cellulose; acrylic polymers such as polymethylmethacrylate; styrene-based polymers such as polystyrene andstyrene-acrylonitrile copolymer (AS resin); polycarbonate-based polymerssuch as a copolymer of bisphenol A and carbonic acid; linear or branchedpolyolefins such as polyethylene, polypropylene, and ethylene-propylenecopolymer; polyolefins including cyclo-structures, such aspolynorbornene; vinyl chloride-based polymers; amide-based polymers suchas nylon and aromatic polyamide; imide-based polymers; sulfone-basedpolymers; polyethersulfone-based polymers; polyether ether ketone-basedpolymers; polyphenylene sulfide-based polymers; vinyl alcohol-basedpolymers; vinylidene chloride-based polymers; vinyl butyral-basedpolymers; arylate-based polymers; polyoxymethylene-based polymers; andepoxy-based polymers. These polymers can be used alone or incombination. Furthermore, suitable additives can be added to thesepolymer materials for arbitrary purposes, e.g., providing extensibilityand contractile property.

In addition, examples of the non-liquid crystal polymers include a resincomposition containing a thermoplastic resin whose side chain hassubstituted or unsubstituted imide group(s) and a thermoplastic resinwhose side chain has substituted or unsubstituted phenyl group(s) andcyano group(s). The example is a resin composition having an alternatingcopolymer including isobutene and N-methylene maleimide and astyrene-acrylonitrile copolymer. Further, for the polyimide-based filmmaterial, for example, materials described in US55880950 and U.S. Pat.No. 5,580,964 can be used suitably for retardation layers composed ofnon-liquid crystal polymers.

Next, since the polarization component of the present invention includesthe two polarizer layers (reflective circular polarizer) thatselectively transmit either clockwise circularly-polarized light orcounterclockwise circularly-polarized light and selectively reflect theother, it is advantageous, for example, in having apolarization-separation function with respect to natural light enteringat a wide angle, and thus it can be designed and manufactured easily.

Though there is no particular limitation about the reflective circularpolarizer, it is further preferable that the planar alignment state ofthe cholesteric liquid crystal is fixed. Though the kind of thecholesteric liquid crystal can be selected suitably without anyparticular limitation, for example, a polymeric liquid crystal formed bypolymerizing liquid crystal monomers, a liquid crystal polymerexhibiting a cholesteric liquid crystal property at a high temperature,and the mixture thereof can be used, as in the case of the retardationlayer. The polymeric liquid crystal can be obtained by adding a chiralagent, an alignment auxiliary or the like to a liquid crystal monomer asrequired, and by polymerizing with ionizing radiation such as anelectron beam and ultraviolet ray or with heat. Though the liquidcrystal property of the cholesteric liquid crystal can be eitherlyotropic or thermotropic, a thermotropic liquid crystal is preferredfrom aspects of easy control and easy formation of monodomain.

Examples of the reflective circular polarizer include, morespecifically, a sheet including a layer of a cholesteric liquid crystalpolymer, a sheet formed by laminating the layer on a glass plate or thelike, and a film of a cholesteric liquid crystal polymer, though thereflective circular polarizers are not limited thereto. Similarly,though methods of forming these cholesteric liquid crystal layers arenot limited particularly, for example, such layers can be formed by amethod similar to the case of a retardation layer including thecholesteric liquid crystal compound. It is more preferable that thecholesteric liquid crystal is aligned as uniformly as possible in thelayer.

Regarding the reflective circular polarizer, it is more preferable thatthe selective reflection wavelength band covers a visible light regionand a band of emission wavelength of a light source, from an aspect ofperformance of the polarization component, and the selective reflectionwavelength band can be determined unequivocally on the basis of thecholesteric chiral pitch and the refractive index of the liquid crystalas described above. Corresponding to the purposes, the cholestericliquid crystal layer composing the reflective circular polarizer can beformed, for example, by laminating a plurality of layers different fromeach other in the selective reflection wavelength bands. Alternatively,the cholesteric liquid crystal layer can be made of a single layer wherethe pitch changes in the thickness direction. For laminating plurallayers, it is also possible to prepare several bases on whichcholesteric liquid crystal layers are laminated previously, which arethen laminated further. However, from an aspect of reducing thethickness or the like, a further preferable method includes forming analignment film on a cholesteric liquid crystal layer on which anothercholesteric liquid crystal layer is laminated.

It is preferable that the polarization component of the presentinvention further includes a layer having a function of a quarterwavelength plate at least in the front direction, and the layer isdisposed outside the reflective circular polarizer that is one of thetwo polarizer layers and that is positioned at the visible side. Due tothe configuration, it is possible to modify a circularly polarized lightbeam passing through the reflective circular polarizer into a linearlypolarized light beam in order to use the light effectively. It isparticularly preferable that the polarization component includes furtheran absorptive dichroic polarizing plate, and the absorptive dichroicpolarizing plate is disposed outside the additional layer having thefunction of a quarter wavelength plate at least in the front direction.

Though the absorptive dichroic polarizing plate is not limitedparticularly, the examples include: absorptive polarizing plates formedby adsorbing a dichroic material such as iodine or a dichroic dye in ahydrophilic polymer film such as a polyvinyl alcohol-based film, apartially-formalized polyvinyl alcohol-based film andpartially-saponified film based on an ethylene-vinyl acetate copolymer,and stretching the same; and a polyene-alignment film made of dehydratedpolyvinyl alcohol or polyvinyl chloride that has been treated toeliminate hydrochloric acid. An alternative example of the polarizingplate is formed by providing a transparent protective layer made of, forexample, a plastic coating layer or a film laminate layer, on at leastone surface of such a film for a purpose of maintaining the water-proofor the like. An alternative example is formed by containing transparentfine particles in the transparent protective layer so as to provide afine irregularity on the surface. The transparent fine particles are,for example, inorganic fine particles having an average particlediameter of 0.5 to 5 μn, such as silica, alumina, titania, zirconia,stannic oxide, indium oxide, cadmium oxide and antimony oxide. Theseparticles can be electroconductive. Furthermore, organic fine particlessuch as crosslinked or uncrosslinked polymers can be used as well.

First Embodiment

Referring to FIGS. 1 and 2, a mechanism of simultaneous expression offocusing and brightness enhancement in a polarization component of thepresent invention will be described below. However, the descriptionbelow refers to only one embodiment of the present invention, and thepresent invention will not be limited by the description.

FIG. 1 shows this embodiment for a polarization component of the presentinvention. As shown in this figure, this polarization component isformed by laminating main elements of a cholesteric liquid crystalcircular polarizer 201 (hereinafter, this may be referred to as ‘layer 1’), a C-plate 202 (hereinafter, this may be referred to as ‘layer 2 ’),and a cholesteric liquid crystal circular polarizer 203 (hereinafter,this may be referred to as ‘layer 3 ’) in this order, and light beamsenter from the layer 1 side. In the embodiment shown in this figure,rotational directions of circularly polarized light beams passingthrough the two reflective circular polarizer layers are the same. Sinceeither the circular polarizer or the retardation layer does not have anyoptical axes present in the in-plane directions, the bonding directionsof the circular polarizer and a retardation layer can be determinedarbitrarily. For this reason, an angular range for limiting the lightparallelizing has an isotropic and symmetric characteristic.

FIG. 2 is an explanatory view showing signs representing natural light,circularly polarized light and linearly polarized light according to thepresent invention. Circularly polarized light ‘a’ and circularlypolarized light ‘b’ rotate in opposite directions, and linearlypolarized light ‘c’ and linearly polarized light ‘d’ cross each other atright angles.

An ideal principle of operation in a case where a light beam enters thepolarization component of FIG. 1 will be explained below sequentially byreferring to the figure.

(1) First, among light beams fed from a backlight (a light source; notshown), a perpendicular-incident natural light beam 1 is polarized andseparated by the circular polarizer 201 (layer 1) into two circularlypolarized light beams, i.e., a transmitted light beam 3 and a reflectedlight beam 2. The rotational directions of the circularly polarizedlight beams are opposite to each other.(2) The transmitted light beam 3 passes through the retardation layer202 (layer 2) and becomes a transmitted light beam 4.(3) The transmitted light beam 4 passes through the circular polarizer203 (layer 3) and becomes a transmitted light beam 5.(4) The transmitted light beam 5 is used for a liquid crystal displayapparatus disposed thereon.(5) Next, among the light beams fed from the backlight, anobliquely-entering natural light beam 6 is polarized and separated bythe circular polarizer 201 into two circularly polarized light beams ofa transmitted light beam 8 and a reflected light beam 7. The circularlypolarized light beams rotate in opposite directions.(6) When passing through the retardation layer 202, the transmittedlight beam 8 is provided with a retardation value of ½ wavelength andbecomes a transmitted light beam 9.(7) Under an influence of the retardation, the rotational direction ofthe circular polarization of the transmitted light beam 9 becomesopposite with respect to the light beam 8.(8) The transmitted light beam 9 is reflected by the circular polarizer203 and becomes a light beam 10.

It is known that a rotational direction of circularly polarized light isreversed generally at reflection (see, for example, “Polarized light:Production and Use”, W. A. Schurcliff, Harvard University Press,Cambridge, Mass., 1966). However, it is also known that, as anexception, the rotational direction will not change in a case ofreflection at a cholesteric liquid crystal layer (see, for example,“Ekisho Jiten (Dictionary for Liquid Crystal)”, published by Baihuukan).Since reflection in this figure is carried out on a cholesteric liquidcrystal surface, the rotational directions of the circularly polarizedlight beams 9 and 10 are not changed.

(9) When passing through the retardation layer 202, the reflected lightbeam 10 is influenced by the retardation and becomes a transmitted lightbeam 11.(10) The rotational direction of the transmitted light beam 11 isreversed under the influence of the retardation.(11) The light beam 11, whose rotational direction becomes identicalagain to that of the light beam 8, passes through the circularlypolarizer 201 and becomes a transmitted light beam 12.(12) The light beam 7 and the light beam 12 are returned to thebacklight side and recycled. These returning light beams are reflectedrepeatedly until they become light beams that can pass through thepolarization component in the vicinity of the normal direction, whilechanging the traveling direction and direction of the polarization atrandom by a diffusion plate or the like disposed on the backlight,thereby contributing enhancement of the brightness.(13) The transmitted circularly polarized light beam 5 is converted intoa linearly polarized light beam by a quarter wavelength plate (notshown), so that it can be used in a liquid crystal display or the likewithout causing any absorption losses. In this manner, focusing andbrightness enhancement by the polarization component of FIG. 1 iscarried out.

Next, a selective reflection wavelength band of the reflective polarizerwill be explained.

The two reflective polarizer layers according to the present inventionhave selective reflection wavelength bands that can be the same ordifferent from each other. For example, though one of the reflectivepolarizers can reflect in a whole band of visible light while the othercan reflect partially, the selective reflection wavelength bands mustoverlap each other at least partially. The selective reflectionwavelength bands of the reflective polarizers can be designedappropriately for example, corresponding to use of the polarizationcomponents and types of members and light sources to be combined in use.It is preferable, for example, that the selective reflection is achievedwith respective to light beams having high visibility in the vicinity ofa wavelength of 550 nm. Specifically, it is preferable that theoverlapping regions of the selective reflection wavelength bands of thetwo reflective polarizer layers include a wavelength range of 540 nm to560 nm. In a case of a reflective polarizer including a cholestericliquid crystal compound, as mentioned above, the selective reflectionwavelength bands can be determined unequivocally on the basis of acholesteric chiral pitch and the refractive index of the liquid crystal;and the central wavelength of the selective reflection is represented byFormula (VIII) (λ=np).

Furthermore, since white light is required when there is a need forcolor display, it is further preferable that the bands have uniformcharacteristics in a visible light band, or that at least an emissionspectrum region of a light source (in many cases, 435 nm to 610 nm) canbe covered. When considering that the selective reflection wavelengthspectrum of the cholesteric liquid crystal is shifted to the shortwavelength side (blue shift) with respect to an oblique incident lightbeam, it is further preferable that the overlapping region covers aregion of a wavelength longer than 610 nm. Since the selectivereflection wavelength band required for this long-wavelength sidedepends considerably on the angle and wavelength of incident light beamsfrom the light source, an end of the long wavelength is set arbitrarilycorresponding to the required specification. Specifically for example,in a backlight using a wedge-type light-guiding plate which is oftenused in a liquid crystal display, light emitted from the light-guidingplate has an angle of about 60° from a normal direction. The amount ofthe above-mentioned blue-shift tends to increase as the incidence angleis larger, and it is about 100 nm at an angle of about 60°. Therefore,when a three-wavelength cold cathode ray tube is used for the backlightand when the red bright-line spectrum is 610 nm, it will be acceptablethat the above-mentioned overlapping region of the selective reflectionwavelength bands reaches closer to the side with a wavelength longerthan 710 nm. Further, from aspects of resolution of coloration and RGBin a liquid crystal display apparatus or the like, it is particularlypreferable that the selective reflection wavelength bands overlap eachother in an entire visible light wavelength region, i.e., a range of 380nm to 780 nm.

When the backlight emits only light of a specified wavelength, e.g., fora case of using a colored cold cathode ray tube, it would be sufficientif only an obtainable bright-line spectrum can be shielded. Furthermore,since transmitted light beams entering at large angles can be ignoredwhen a light beam emitted from the backlight is stopped down to somedegrees from the beginning in the front direction due to the design ofthe microlens, dot, and prism processed on the moving subject, there isno need for extending the selective reflection wavelength considerablyto the long wavelength side.

Next, a retardation value for the retardation layer will be described.

The oblique direction retardation value R′ of the retardation layer (seeFormula (II) above) should be λ/2 (λ denotes a wavelength of incidentlight) ideally for the reflective polarizer to entirely reflect a lightbeam passing through the retardation layer, but in fact, the object canbe achieved even if the value is not λ/2 strictly. Furthermore, sincethe oblique direction retardation value R′ is changed due to theincidence angle, that is, it tends to be increased in general with anincrease of the incidence angle, for causing efficient conversion inpolarization, the retardation value R′ must be determined suitably byconsidering the angle for entire reflection or the like. For example,for an entire reflection at an angle formed by a normal being about 60°,the value should be set so that the retardation at a measurement at 60°will be about λ/2. There is no particular limitation about the methodfor adjusting the oblique direction retardation value R′, and knownmethods can be applied suitably. For example, when the retardation layeris a biaxially stretched film, the adjustment can be conducted using thestretching ratio, film thickness or the like.

Furthermore, a light beam transmitted through the reflective polarizermay change its polarization state due to the birefringence or the likeof the reflective polarizer itself that functions like a C-plate. Forexample, a reflective circular polarizer including a cholesteric liquidcrystal layer can have some properties of a retardation layer such as anegative C-plate due to the twist structure of the cholesteric liquidcrystal compound. Therefore, it is possible to adjust the obliquedirection retardation value R′ of the retardation layer to be smallerthan λ/2 by considering the retardation of the reflective polarizer.Specifically, R′ should be at least λ/8 as represented in Formula (II).The upper limit of the R′ is not determined particularly, and it can beset suitably corresponding to the object. As mentioned above, thein-plane retardation R (see Formula (I)) is preferred to be smaller.

For reference, FIG. 10 shows an index ellipsoid representing clearly arelationship between an incidence angle of a C-plate and retardation,and the optical anisotropy of the C-plate. However, this is just anexample, and thus it does not intend to limit the present invention.FIG. 10 shows an example where the birefringent resin has biaxialalignment of front retardation≈0 and oblique retardation=½ wavelength,and in this case, a ½ wavelength is obtained at a position of ±40degrees.

The embodiment as described above regarding a use of a reflectivecircular polarizer is not limited thereto, but can be modified in manyways. For example, in the present invention, the C-plate used for theretardation layer can be replaced by a half wavelength plate (alsoreferred to as a half wavelength retardation plate). That is, apolarization component of the present invention may include at least tworeflective circular polarizer layers and a half wavelength platedisposed between the reflective circular polarizer layers, wherein thereflective circular polarizer layers have selective reflectionwavelength bands for selective reflection of polarized light overlappingeach other at least partially. In this case, it is preferable that therotational directions of circularly polarized light beams passingthrough each of the two reflective circular polarizer layers areopposite each other, and it is ideal that the oblique directionretardation value in the half wavelength plate is 0 or λ. When settingthe oblique direction retardation value, the retardation value of thereflective circular polarizer layer should be considered as in the caseof using a C-plate. When using the half wavelength plate, problems ofanisotropy or coloration may occur depending on azimuth of the inclinedaxis. However, for example, such coloration can be compensated, for therespective layers of the two reflective circular polarizer layers andthe retardation layer, by using layers different from each other in thewavelength diffusion properties.

Second Embodiment

Another embodiment of the present invention will be described below.

In the polarization component of the present invention, the reflectivepolarizer can be a reflective linear polarizer. More specifically, apolarization component of the present invention may include at least tworeflective polarizer layers and an intermediate layer disposed betweenthe reflective polarizer layers,

the two reflective polarizer layers are reflective linear polarizerlayers that selectively transmit one of linearly polarized light beamscrossing each other at right angles while selectively reflecting theother,

the two reflective linear polarizer layers have selective reflectionwavelength bands for selective reflection of polarized light, the bandsoverlapping each other at least partially,

the intermediate layer comprises a single optical layer or a laminate ofat least two optical layers, and the intermediate layer has a functionof transmitting an incident linearly polarized light beam while changingor not changing the polarization direction, according to the incidencedirection,

the two reflective linear polarizer layers are disposed at an angle soas to have in-plane slow axis directions for transmitting a light beamthat is included in incident linearly polarized light and that enters ina direction (normal direction) perpendicular to the incidence surfacewhile efficiently reflecting a light beam entering from an obliquedirection.

Such a polarization component can be prepared, for example, by combininga reflective linear polarizer and a quarter wavelength plate (alsoreferred to as a quarter wavelength retardation plate) to sandwich aC-plate. More specifically, a preferable polarization component includesat least two reflective linear polarizer layers, and a retardation layerand two quarter wavelength plate layers both of which are disposedbetween the reflective linear polarizer layers; wherein one of thequarter wavelength plate layers is disposed between one of thereflective linear polarizer layers and the retardation layer, and theother quarter wavelength plate is disposed between the other reflectivelinear polarizer layer and the retardation layer; the two reflectivelinear polarizer layers have selective reflection wavelength bands forselective reflection of polarized light, the selective reflectionwavelength bands overlapping each other at least partially, and thequarter wavelength plate positioned on one of the surfaces of theretardation layer has an in-plane slow axis forming an angle of 40° to50° with respect to the polarization axis of the reflective linearpolarizer layer positioned on the same side of the retardation layer;the other quarter wavelength plate positioned on the other surface ofthe retardation layer has to an in-plane slow axis forming an angle of−40° to −50° with respect to the polarization axis of the reflectivelinear polarizer layer positioned on the same side of the retardationlayer; and an angle formed by the in-plane slow axes in the tworeflective linear polarizer layers is determined arbitrarily. In thiscase, the retardation layer must satisfy the following Formulae (I) and(III).

R≧(λ/10)  (I)

R′≧(λ/4)  (III)

Definitions of λ, R and R′ in Formulae (I) and (III) are describedabove.

It has been known that natural light can be converted to circularlypolarized light by a combination of a linear polarizer and a quarterwavelength plate. As shown in FIG. 3, a natural light beam 301 enteringa linear polarizer 302 is converted to a linearly polarized light beam303, and further the linearly polarized light beam 303 passing through aquarter wavelength plate 304 is converted to a circularly polarizedlight beam 305. The reflective circular polarizer layer and thereflective linear polarizer layer are advantageous in comparison with aprism type reflective polarizer based on principles such as Brewster'sangle in that it is less dependent on the incidence angle.

When simply sandwiching a C-plate by reflective linear polarizer layers,an optical axis with respect to light beams entering the C-plate from anoblique direction will cross the direction of light beam always at rightangles, and thus no retardation will be expressed, and polarizationconversion will not occur. In such a case, a linearly polarized lightbeam is converted to a circularly polarized light beam by a quarterwavelength plate having a slow axis direction of 45° or −45° withrespect to a polarization axis of the reflective linear polarizer layer,and subsequently, converted to a reverse circularly polarized light beamby using retardation of the C-plate, which is then converted again to alinearly polarized light beam by a quarter wavelength plate.

The quarter wavelength plate and the half wavelength plate according tothe present invention are not limited particularly, and any known platescan be used suitably. Specific examples include uniaxially-stretched orbiaxially-stretched polymer films, layers formed by hybrid-aligning(i.e., uniaxially aligning in the plane direction and further aligningin the thickness direction) liquid crystal compounds, and the like.There is no particular limitation about the method of controlling thein-plane retardation in the quarter wavelength plate and the halfwavelength plate, and for example, a stretched polymer film can becontrolled by adjusting the stretch ratio, film thickness and the like.

Though polymers that can be used for the polymer films are not limitedparticularly, preferable examples include: polyester-based polymers suchas polyethylene terephthalate and polyethylene naphthalate;cellulose-based polymers such as diacetyl cellulose and triacetylcellulose; acrylic polymers such as polymethyl methacrylate;styrene-based polymers such as polystyrene and styrene-acrylonitrilecopolymer (AS resin); polycarbonate based polymers such as a copolymerof bisphenol A and carbonic acid; linear or branched polyolefins such aspolyethylene, polypropylene, and ethylene-propylene copolymer;polyolefins including cyclo-structures, such as polynorbornene; vinylchloride-based polymers; amide-based polymers such as nylon and aromaticpolyamide; imide-based polymers; sulfone-based polymers;polyethersulfone-based polymers; polyether ether ketone-based polymers;polyphenylene sulfide-based polymers; vinyl alcohol-based polymers;vinylidene chloride-based polymers; vinyl butyral-based polymers;arylate-based polymers; polyoxymethylene-based polymers; and epoxy-basedpolymers. These polymers can be used alone or in combination.Furthermore, suitable additives can be added to these polymer materialsfor arbitrary purposes, e.g., providing extensibility and contractileproperty.

Similarly, methods of manufacturing the polymer films are not limitedparticularly, and polymer films manufactured by casting (extrusion) andpolymer films manufactured by melting the polymer material, shaping to afilm and stretching, can be used. The latter example is preferred froman aspect of mechanical strength or the like.

Another example of the polymer film is described in JP 2001-343529 A(WO01/37007). For the materials of the polymer film, for example, aresin composition containing a thermoplastic resin whose side chain hassubstituted or unsubstituted imide group(s) and a thermoplastic resinwhose side chain has substituted or unsubstituted phenyl group(s) andcyano group(s) can be used. The example is a resin composition having analternating copolymer including isobutene and N-methylene maleimide anda styrene-acrylonitrile copolymer.

Similarly, a reflective linear polarizer of the present invention is notlimited particularly, and known products can be used. Specifically forexample, a stretched film having an optical anisotropy, a laminatethereof and the like can be used, and the material of the stretched filmcan be similar to those of the quarter wavelength plate and the halfwavelength plate.

FIG. 4 is a schematic view showing a polarization component of thisembodiment, though the present embodiment will not be limited to thisexample. As shown in this figure, this polarization component is formedby laminating main elements, i.e., a reflective linear polarizer 404(hereinafter this may be called ‘layer 4 ’), a quarter wavelength plate405 (hereinafter this may be called ‘layer 5 ’), a C-plate 406(hereinafter this may be called ‘layer 6 ’), a quarter wavelength plate407 (hereinafter this may be called ‘layer 7’), and a reflective linearpolarizer 408 (hereinafter this may be called ‘layer 8’) in this order,and light enters from the layer 4 side.

FIG. 5 is a schematic view to show angles for bonding the respectivemain elements in the polarization component of FIG. 4. An angle formedby the polarization axis of the linear polarizer 404 and the in-planeslow axis of the quarter wavelength plate 405 is in a range of 40° to50°, and an angle formed by the polarization axis of the linearpolarizer 408 and the in-plane slow axis of the quarter wavelength plate407 is in a range of −40° to −50°. Excepting this, there is noparticular limitation about angles formed by the respective elements,and similar performance can be obtained even by arbitrarily rotating aset 1 (a combination of the linear polarizer 404 and the quarterwavelength plate 405) and a set 2 (a combination of the linear polarizer408 and the quarter wavelength plate 407) while maintaining theabove-mentioned angles. For example, FIG. 14 shows an example where theset 2 as shown in FIGS. 4 and 5 is rotated by 90°. Even this example canexhibit performance just as shown in FIGS. 4 and 5. Since the C-platedoes not have an optical axis within the plane, the angle for bondingcan be decided arbitrarily.

An ideal operation principle in a case where a light beam enters thepolarization component of the present embodiment will be describedbelow, by referring to FIG. 4.

(1) First, a natural light beam 14 is emitted from a backlight (lightsource) so as to enter the reflective linear polarizer 404 (layer 4)perpendicularly.(2) The light beam 14 is separated into a linearly polarized light beam15 and a linearly polarized light beam 16 crossing at right angles. Thelight beam 15 passes through the layer 4, and the light beam 16 isreflected.(3) The linearly polarized light beam 5 passes through the quarterwavelength plate 405 (layer 5) and is converted to a circularlypolarized light beam 17.(4) The circularly polarized light beam 17 passes through the C-plate406 (layer 6) as a circularly polarized light beam 18, without changingits polarization state.(5) The circularly polarized light beam 18 passes through the quarterwavelength plate 407 (layer 7) and is converted to a linearly polarizedlight beam 19.(6) The linearly polarized light beam 19 passes through the C-plate 408(layer 8) as a linearly polarized light beam 20, without changing itspolarization state.(7) The linearly polarized light beam 20 enters an apparatus (a liquidcrystal display apparatus or the like), and is transmitted withoutlosses.(8) From the backlight, in addition to the natural light beam 14 from aperpendicular direction, a natural light beam 21 is emitted to enterobliquely the layer 4.(9) The light beam 21 is separated into a linearly polarized light beam22 and a linearly polarized light beam 23 crossing at right angles. Thelight beam 22 passes through the layer 4 (reflective linear polarizer),and the light beam 23 is reflected.(10) The linearly polarized light beam 22 passes through the layer 5(reflective linear polarizer) and is converted to a circularly polarizedlight beam 24.(11) The circularly polarized light beam 24 is subjected to retardationof ½ wavelength when passing through the layer 6 (C-plate) so as toreverse the rotation direction and become a circularly polarized lightbeam 25.(12) The circularly polarized light beam 25 passes through the layer 7(a quarter wavelength plate) and is converted to a linearly polarizedlight beam 26.(13) The linearly polarized light beam 26 is reflected by the layer 8(reflective linear polarizer) and becomes a linearly polarized lightbeam 27.(14) The linearly polarized light beam 28 passes through the layer 7 (aquarter wavelength plate) and is converted to a circularly polarizedlight beam 28.(15) The circularly polarized light beam 28 is subjected to retardationof a half wavelength plate when passing through the layer 6 (C-plate) soas to reverse the rotation direction and become a circularly polarizedlight beam 29.(16) The circularly polarized light beam 29 passes through the layer 5(a quarter wavelength plate) and is converted to a linearly polarizedlight beam 30.(17) The linearly polarized light beam 30 passes through the layer 4(reflective linear polarizer) as a linearly polarized light beam 31,without changing its polarization state.(18) The reflected light beams 16, 23 and 31 are returned to thebacklight side and recycled. The recycling mechanism is the same as thatof First Embodiment.

In this embodiment, the angle formed by the polarization axis of thereflective linear polarizer and the in-plane slow axis of the quarterwavelength plate in the set 1 and set 2 (FIG. 5) should be 45° and −45°theoretically in an ideal system. However, since the characteristics ofthe reflective polarizer and wavelength plate are not perfect in fact inthe visible light range and there are subtle changes for respectivewavelengths, problems such as coloration can occur. Such problems likecoloration can be solved by compensating the color tone by shifting theangle to some degrees so as to optimize the entire system rationally.Since other problems such as degradation in the transmittance may occurwhen the angle is considerably out of 45° or −45°, the adjustment willbe limited to a range of ±5°

A preferable range for the selective reflection wavelength band of thereflective linear polarizer is similar to that of the reflectivecircular polarizer. Just as the case of the reflective circularpolarizer, the wavelength property of a transmitted light beam isshifted toward the short wavelength side with respect to a light beamentering in the oblique direction, preferably it has a sufficientpolarization property and a retardation property in the long wavelengthside out of the visible light region, so that the reflective linearpolarizer functions sufficiently with respect to a light beam enteringat a deep angle.

Furthermore, for the preferable range of the oblique directionretardation value R′ (Formula (III)) in a retardation layer (C-plate) inthis embodiment, it can be adjusted on the basis of the idea similar tothe case of using a reflective circular polarizer. However, since atypical reflective linear polarizer has itself a smaller retardationproperty in comparison with a reflective circular polarizer, the R′ ofat least ⅛ wavelength is insufficient, but it should be ¼ wavelength orlarger.

In FIG. 15, a change in polarization state caused by a quarterwavelength plate, a C-plate and another quarter wavelength plate, whichare placed between two reflective polarizers, is indicated on a Poincaresphere, for a case where an oblique incident light beam enters thepolarization component of FIG. 14. The figure shows that a linearlypolarized light beam entering from the first reflective polarizer isconverted to a circularly polarized light beam and to a reversedlinearly polarized light beam. However, this is an illustration to showone example of the present invention, not to limit the presentinvention.

Third Embodiment

Another embodiment of the present invention will be described below.

Similar effects can be obtained in this embodiment by laminating, atright angles or in parallel, two biaxial films whose front retardation(in-plane retardation) is λ/4 and whose thickness direction retardationis at least λ/2, instead of using a structure formed by sandwiching aC-plate with two quarter wavelength plate layers. In this case, a Nzcoefficient (thickness direction retardation/in-plane retardation) of 2or more can satisfy the requirement.

That is, a polarization component of the present invention may includeat least two reflective linear polarizer layers and two quarterwavelength plate layers disposed between the reflective linear polarizerlayers, wherein

the two reflective linear polarizer layers have selective reflectionwavelength bands for selective reflection of polarized light, theselective reflection wavelength bands overlapping each other at leastpartially,

an in-plane slow axis of one of the quarter wavelength plates forms anangle of 40° to 50° with respect to a polarization axis of thereflective linear polarizer layer positioned on the same side of thepolarization component,

an in-plane slow axis of the other quarter wavelength plate forms anangle of −40° to −50° with respect to a polarization axis of thereflective linear polarizer layer on the same side of the polarizationcomponent,

an angle formed by the in-plane slow axes of the two quarter wavelengthplate layers is determined arbitrarily, and

the quarter wavelength plate layers satisfy respectively the conditionof Formula (IV) below:

Nz≧2.0  (IV)

where Nz=(nx−nz)/(nx−ny).

In Formula (IV), nx, ny and nz are respectively refractive indices inX-axis, Y-axis and Z-axis directions in the quarter wavelength plate,where the X-axis direction is a direction showing a maximum refractiveindex within the plane of the quarter wavelength plate (in-plane slowaxis direction), the Y-axis direction is a direction perpendicular tothe X-axis within the plane of the quarter wavelength plate (in-planefast axis direction), and the Z-axis direction is a thickness directionof the quarter wavelength plate and perpendicular to the X-axisdirection and to the Y-axis direction.

There are no specific limitations about the materials of the quarterwavelength plate and the reflective linear polarizer layer, or themethods of controlling the in-plane retardation and the thicknessdirection retardation, and description in Second Embodiment can beapplied.

FIG. 6 is a schematic view showing a polarization component of thisembodiment, though the present embodiment is not limited thereto. Asshown in this figure, this polarization component is formed bylaminating main elements, i.e., a reflective linear polarizer 609(hereinafter this may be called ‘layer 9 ’), a quarter wavelength plate610 (hereinafter this may be called ‘layer 10 ’), a quarter wavelengthplate 611 (hereinafter this may be called ‘layer 11 ’), and a reflectivelinear polarizer 612 (hereinafter this may be called ‘layer 12 ’) inthis order, and light enters from the layer 9 side.

FIG. 7 is a schematic view to show angles for bonding the respectivemain elements in the polarization component of FIG. 6. An angle formedby the polarization axis of the linear polarizer 609 and the in-planeslow axis of the quarter wavelength plate 610 is in a range of 40° to50°, and an angle formed by the polarization axis of the linearpolarizer 612 and the in-plane slow axis of the quarter wavelength plate611 is in a range of −40° to −50°. Excepting this, there is noparticular limitation about angles formed by the respective elements,and similar performance can be obtained even by arbitrarily rotating aset 1 (a combination of the linear polarizer 609 and the quarterwavelength plate 610) and a set 2 (a combination of the linear polarizer612 and the quarter wavelength plate 611) while maintaining theabove-mentioned angles. For convenience in explanation, FIGS. 6 and 7show an example in which axes of the upper and lower linear polarizersare parallel and the axes of the quarter wavelength plate layers crosseach other at right angles, but the example is non-limiting.

An ideal operation principle in a case where a light beam enters thepolarization component of the present embodiment will be describedbelow, by referring to FIG. 6.

(1) First, a natural light beam 32 emitted from a backlight (lightsource) enters perpendicularly.(2) The natural light beam 32 is separated by the layer 9 (reflectivelinear polarizer) into a linearly polarized light beam 33 and a linearlypolarized light beam 34 crossing at right angles. The linearly polarizedlight beam 33 passes through the layer 9, and the linearly polarizedlight beam 34 is reflected.(3) The linearly polarized light beam 33 passes through the layer 10 andthe layer 11 (quarter wavelength plates). Since the in-plane slow axesof the layer 10 and of the layer 11 cross at right angles in the exampleas shown in this figure, when considering a combination of the layer 10and the layer 11, the front retardation (in-plane retardation) becomes0. As a result, when passing through the layers 10 and 11, the linearlypolarized light beam 33 becomes a linearly polarized light beam 35without changing its polarization state.(4) The linearly polarized light beam 35 passes through the layer 12(reflective linear polarizer) without changing its polarization state,and becomes a linearly polarized light beam 36.(5) The linearly polarized light beam 36 is transmitted to an apparatus(e.g., a liquid crystal display apparatus), without losses.(6) From the backlight, in addition to the natural light beam 32 thatenters perpendicularly, a natural light beam 37 is emitted to enterobliquely.(7) The natural light beam 37 is separated by the layer 9 (reflectivelinear polarizer) into a linearly polarized light beam 38 and linearlypolarized light beam 39 crossing at right angles. The linearly polarizedlight beam 38 passes through the layer 9, and the linearly polarizedlight beam 39 is reflected.(8) The linearly polarized light beam 38 enters obliquely the layer 10and the layer 11, and when passing through these layers, it becomes alinearly polarized light beam 40 as the polarization axis directionchanges by 90° under an influence of the thickness directionretardation.(9) The linearly polarized light beam 40 enters the layer 12 (reflectivelinear polarizer).(10) Since the layer 12 has the same axial direction as the layer 9, thelinearly polarized light beam 40 is reflected by the layer 12 andbecomes a linearly polarized light beam 41.(11) The linearly polarized light beam 41 is subjected to an influenceby retardation as mentioned in the above (9) when passing through thelayer 10 and the layer 11, thereby changing the polarization axisdirection by 90° so as to become a linearly polarized light beam 42.(12) The linearly polarized light beam 42 passes through the layer 9(reflective linear polarizer) without changing its polarization state,and becomes a linearly polarized light beam 43.(13) The reflected light beams 34, 39 and 43 are returned to thebacklight side and recycled. The recycling mechanism is the same asdescribed in First and Second Embodiments.

The polarization component of this embodiment can exhibit a performancesimilar to that of the polarization component of Second Embodiment.Furthermore, since the C-plate can be omitted, the polarizationcomponent in this embodiment is superior to the polarization componentof Second Embodiment in the production efficiency. Though there is noparticular limitation about the quarter wavelength plate in thisembodiment and the above-mentioned one can be used, for example, filmsof a biaxially-stretched polycarbonate (PC) and polyethyleneterephthalate (PET) or a layer of hybrid-aligned liquid crystal compoundare further preferred.

Regarding the range of angles formed by the reflective linear polarizerand the quarter wavelength plate, the above description can be applied,and it can be slightly adjusted on the basis of the concept as in theSecond Embodiment.

Similarly, regarding the selective reflection wavelength band of thereflective linear polarizer, the First and Second Embodiments can beapplied.

Furthermore in this embodiment, the use efficiency of an obliqueincident light beam is changed by changing the value of Nz (Formula(IV)). The preferable range is not limited particularly, and it can beadjusted to obtain an optimum efficiency for light utilization on thebasis of a concept as in First and Second Embodiments. This embodimentis similar to the respective embodiments mentioned above in that theretardation of the reflective polarizer must be taken intoconsideration.

Fourth Embodiment

Another embodiment of the present invention will be described below.

Similar effects can be obtained by using a biaxial film whose frontretardation (in-plane retardation) is λ/2 and whose thickness directionretardation is at least λ/2, instead of a structure formed bysandwiching a C-plate with two quarter wavelength plate layers as inSecond Embodiment. In this case, a Nz coefficient of 1.5 or more cansatisfy the requirement.

That is, a polarization component of the present invention may includeat least two reflective linear polarizer layers and a half wavelengthplate disposed between the reflective linear polarizer layers, wherein

the two reflective linear polarizer layers have selective reflectionwavelength bands for selective reflection of polarized light, theselective reflection wavelength bands overlapping each other at leastpartially,

the in-plane slow axis of the half wavelength plate forms an angle of40° to 50° with respect to a polarization axis of one of the reflectivelinear polarizer layers, and also forms an angle of −40° to −50° C. withrespect to a polarization axis of the other reflective linear polarizerlayer, and

the half wavelength plate satisfies Formula (V) below:

Nz≧1.5  (V)

where Nz=(nx−nz)/(nx−ny).

In Formula (V), nx, ny and nz are respectively refractive indices inX-axis, Y-axis and Z-axis directions in the half wavelength plate, wherethe X-axis direction is a direction showing a maximum refractive indexwithin the plane of the to half wavelength plate (in-plane slow axisdirection), the Y-axis direction is a direction perpendicular to theX-axis direction within the plane of the half wavelength plate (in-planefast axis direction), and the Z-axis direction is a thickness directionof the half wavelength plate and perpendicular to the X-axis directionand to the Y-axis direction.

There are no specific limitations about the materials and methods ofmanufacturing the reflective linear polarizer and the wavelength plate,and descriptions in previous embodiments can be applied.

FIG. 8 is a schematic view showing a polarization component of thisembodiment, though the present embodiment is not limited thereto. Asshown in this figure, this polarization component is formed bylaminating main elements, i.e., a reflective linear polarizer 813(hereinafter this may be called ‘layer 13 ’), a half wavelength plate814 (hereinafter this may be called ‘layer 14 ’), and a reflectivelinear polarizer 815 (hereinafter this may be called ‘layer 15 ’) inthis order, and light enters from the layer 13 side.

FIG. 9 is a schematic view to show angles for bonding the respectivemain elements in the polarization component of FIG. 8. An angle formedby the polarization axis of the linear polarizer 813 and the in-planeslow axis of the half wavelength plate 814 is in a range of 40° to 50°,and an angle formed by the polarization axis of the linear polarizer 815and the in-plane slow axis of the half wavelength plate 814 is in arange of −40° to −50°. Therefore, the in-plane slow axes of the twolinear polarizer layers will cross necessarily at substantially rightangles.

The polarization component of this embodiment can exhibit performancejust as the polarization components of Second and Third Embodiments, andit is further advantageous in terms of production efficiency because thenumber of laminations is decreased.

An ideal operation principle in a case where a natural light beam entersthe polarization component of the present embodiment will be describedbelow, by referring to FIG. 8.

(1) First, a natural light beam 47 emitted from a backlight (lightsource) enters perpendicularly.(2) The natural light beam 47 is separated by the layer 13 into alinearly polarized light beam 48 and a linearly polarized light beam 49crossing at right angles. The linearly polarized light beam 48 passesthrough the layer 13, and the linearly polarized light beam 49 isreflected.(3) When passing through the layer 14 (half wavelength plate), thelinearly polarized light beam is subjected to an influence of frontretardation (in-plane retardation), thereby rotating its polarizationaxis direction by 90° so as to become a linearly polarized light beam50.(4) The linearly polarized light beam 50 passes through the layer 15(reflective linear polarizer) without changing its polarization state,and becomes a linearly polarized light beam 51.(5) The linearly polarized light beam 51 is transmitted to an apparatus(e.g., a liquid crystal display apparatus), without losses.(6) From the backlight, in addition to the natural light beam 47 thatenters perpendicularly, a natural light beam 52 is emitted to enterobliquely.(7) The natural light beam 52 is separated by the layer 13 (reflectivelinear polarizer) into a linearly polarized light beam 53 and a linearlypolarized light beam 54 crossing at right angles. The linearly polarizedlight beam 53 passes through the layer 13, and the linearly polarizedlight beam 54 is reflected.(8) The linearly polarized light beam 53 enters obliquely the layer 14(a half wavelength plate), and passes as a linearly polarized light beam55 through the layer 14 without changing its polarization axisdirection.(9) The linearly polarized light beam 55 is reflected by the layer 15(reflective linear polarizer) and becomes a linearly polarized lightbeam 56.(10) The linearly polarized light beam 56 enters the layer 14, passingthrough without changing its polarizing axis direction, and becomes alinearly polarized light beam 57.(11) The linearly polarized light beam 57 passes through the layer 13without changing its polarization, and becomes a linearly polarizedlight beam 58.(12) The reflected light beams 49, 54 and 58 are returned to thebacklight side and recycled. The recycling mechanism is the same asdescribed in the former embodiments.

The range of angles formed by a reflective linear polarizer and a halfwavelength plate is as mentioned above. Fine adjustment of the range ofangles formed by the reflective linear polarizer and the half wavelengthplate can be carried out according to the same concept as in the Secondand Third Embodiments.

Similarly, First to Third Embodiments can be applied regarding theselective reflection wavelength band of the reflective linear polarizer.

Furthermore in this embodiment, the use efficiency of the obliqueincident light beam is changed by changing the value of Nz (Formula(V)). The preferable range is not limited particularly, and it can beadjusted to obtain an optimum efficiency of light on the basis of thesame concept as First to Third Embodiments. This embodiment is similarto the respective embodiments mentioned above in that the retardation ofthe reflective polarizer must be taken into consideration.

Though the present invention is described above regarding First toFourth Embodiments, the present invention is not limited to the abovedescription but can be modified variously within a range not deviatingfrom the scope of the present invention. For example, the polarizationcomponent of the present invention can include, in addition to therespective elements as described above, any other optical layers or anyother elements within the scope for achieving an object of theinvention.

(Manufacturing Method and so on)

Next, a method and so on for manufacturing a polarization component ofthe present invention will be described. Materials and methods formanufacturing respective elements such as the C-plate, the reflectivepolarizer and the wavelength plate are as described above.

Though the method of manufacturing a polarization component of thepresent invention is not limited particularly, the polarizationcomponent can be manufactured by laminating the respective elementsmentioned above and any other elements as required. Though the form oflamination is not limited particularly and the respective elements canonly be superposed, the elements are preferably laminated viatranslucent adhesives or pressure-sensitive adhesives from aspects ofthe workability and efficiency of light. Though there is no specificdistinction between “an adhesive” and “a pressure-sensitive adhesive” inthe present invention, adhesives that can be peeled and bonded againcomparatively easy is called “a pressure-sensitive adhesive” forconvenience.

Though there is no particular limitation, from aspects of suppression ofsurface reflection and the like, it is preferable that the adhesive orthe pressure-sensitive adhesive is transparent and has no absorption inthe visible light range, and that the refractive index is close to thoseof the respective layers as much as possible. Therefore, for example,adhesives or pressure-sensitive adhesives based on acrylic, epoxy andisocyanate substances can be used preferably. These adhesives orpressure-sensitive adhesives suitably used can be a solvent type, or forexample, an ultraviolet polymerization type, a thermal polymerizationtype, a two-liquids mixture type or the like. The method of laminatingthe respective elements can be selected suitably depending on thecharacteristics, without any particular limitations. For example, layerscan be laminated in a certain order by a method of forming mono-domainsseparately on alignment layer or the like and transferring it to atranslucent base.

For example, when the respective elements are layers containing liquidcrystal compounds, it is also possible to form suitably alignment layersor the like instead of using layers of an adhesive or apressure-sensitive adhesive, and to form directly the respectiveelements in a certain order (which is called ‘direct and continuouscoating). This method is advantageous from aspects of, for example,decreasing thickness of the polarization component. When using areflective circular polarizer and a C-plate, since the respectiveelements have no optical axes within the plane and since the bondingangle can be determined arbitrarily, bonding through a roll-to-rollprocess and the above-mentioned direct-continuous coating can be used ina manufacturing process, therefore the productivity is improved.

The respective elements and the adhesive layer (pressure-sensitiveadhesive layer) can include further various additives or the like asrequired. For example, particles for diffusivity adjustment can be addedfor providing isotropic dispersion, or surfactants or the like can beadded suitably for providing a leveling property at the time of filmformation. Other than that, ultraviolet absorbers or antioxidants or thelike can be added suitably.

(Polarization Light Source and Image Display Apparatus)

Next, a polarization light source and an image display apparatus using apolarization component of the present invention will be described below.

A polarization light source (polarization light source apparatus) of thepresent invention includes a light source, a reflective layer and apolarization component of the present invention, and this polarizationcomponent is laminated on the light source via the reflective layer.Though the methods for manufacturing a polarization light source are notlimited specifically, for example, a method described in JP H10(1998)-321025 A can be used.

Furthermore, an image display apparatus of the present invention is animage display apparatus including a polarization component of thepresent invention. Though the image display apparatus using thepolarization component or the polarization light source of the presentinvention can be used preferably to image display apparatuses such asorganic EL display apparatuses, PDPs, and CRTs without any particularlimitations, it can be used particularly preferably in a liquid crystaldisplay apparatus.

A liquid crystal display apparatus of the present invention will bedescribed below.

A liquid crystal display apparatus of the present invention includes thepolarization light source of the present invention, and a liquid crystalcell is laminated further on the polarization light source. There is noparticular limitation about elements and methods of manufacturing theliquid crystal display apparatus of the present invention, and publiclyknown elements and manufacturing methods can be used suitably. Thepolarization light source of the present invention is excellent in lightefficiency, and thus it can provide bright light that is excellent inperpendicularity of emitted light and free of brightness nonuniformity.Moreover, the polarization light source can be upsized. Therefore, thepolarization light source can be used preferably as a backlight systemin formation of various liquid crystal display apparatuses, andparticularly, it can be used preferably for a direct-view type liquidcrystal display apparatus.

Regarding a liquid crystal cell to be used in the liquid crystal displayapparatus of the present invention, any suitable one can be used withoutany particular limitations. Particularly, a liquid crystal cell thatallows light beams in a polarization state to enter for providing adisplay is used suitably. For example, liquid crystal cells using atwist nematic liquid crystal or a supertwist nematic liquid crystal arepreferred. However, without being limited to these examples, liquidcrystal cells that use non-twist liquid crystal, a guest-host baseliquid crystal having a dichroic dye dispersed in a liquid crystal, aferroelectric liquid crystal or the like are suitable as well.Similarly, there is no particular limitation about the system fordriving the liquid crystal.

There is no particular limitation about elements other than the liquidcrystal cell, and known members for a liquid crystal display apparatuscan be used suitably. For example, suitable optical layers can bedisposed suitably, and the optical layers include a diffusion plate, ananti-glare layer, an antireflection film, a protective layer, aprotective plate, any of which will be disposed on a polarizing plate ofthe viewing side, and also a compensating retardation plate that will bedisposed between a liquid crystal cell and a polarizing plate.

Next, an organic electroluminescence apparatus (organic EL displayapparatus) will be described below.

Though the polarization component and the polarization light source ofthe present invention can be used for various image display apparatusesother than liquid crystal display apparatuses, they are suitable, forexample, for organic EL display apparatuses. Regarding the organic ELdisplay apparatus of the present invention, there is no specificlimitation other than use of the polarization component or thepolarization light source of the present invention, and publicly knownconfigurations and manufacturing methods can be used. The followingexplanation about an organic EL display apparatus is not for limitingthe present invention.

In general, an organic EL display apparatus has a luminant (organicelectroluminescent luminant) that is prepared by laminating, on atransparent substrate, a transparent electrode, an organic luminantlayer and a metal electrode in a certain order. Here, the organicluminant layer is a laminated body of various organic thin films. Knownexamples thereof include a laminate of a hole injection layer made oftriphenylamine derivative(s) or the like and a luminant layer made of afluorescent organic solid such as anthracene; a laminate of the luminantlayer and an electron injection layer made of perylene derivative or thelike; or a laminate of the hole injection layer, the luminant layer andthe electron injection layer.

In general, the organic EL display apparatus emits light on theprinciple of a system of applying a voltage to the transparent electrodeand the metal electrode so as to inject holes and electrons into theorganic luminant layer, energy generated by the re-bonding of theseholes and electrons excites the fluorescent substance, and the excitedfluorescent substance emits light when it returns to the basis state.The mechanism of the re-bonding during the process is similar to that ofan ordinary diode. This implies that current and the light emittingintensity exhibit a considerable nonlinearity accompanied with arectification with respect to the applied voltage.

It is preferred for the organic EL display apparatus that at least oneof the electrodes is transparent so as to obtain luminescence at theorganic luminant layer. In general, a transparent electrode of atransparent conductive material such as indium tin oxide (ITO) is usedfor the anode. Use of substances having small work function for thecathode is effective for facilitating the electron injection and therebyraising luminous efficiency, and in general, metal electrodes such asMg—Ag, and Al—Li may be used.

In an organic EL display apparatus configured as described above, theorganic luminant layer is made of a film that is extremely thin such asabout 10 nm. Therefore, the organic luminant layer can transmitsubstantially whole light like the transparent electrode does. As aresult, when the layer does not illuminate, incident light from thesurface of the transparent substrate and passing through the transparentelectrode and the organic luminant layer before being reflected at themetal layer comes out again to the surface of the transparent substrate.Thereby, the display surface of the organic EL display apparatus lookslike a mirror when viewed from the exterior.

As mentioned above, a typical organic EL display apparatus has anorganic luminescent layer that emits light when applied with voltage,and the organic luminescent layer has a transparent electrode on thesurface side and a metal electrode on the backside. The organicluminescent layer, the transparent electrode and the metal electrode areintegrated to form an organic electro-luminant. In such an organic ELdisplay apparatus, a polarizing plate can be disposed on the surfaceside of the transparent electrode and at the same time, a retardationplate can be disposed between the transparent electrode and thepolarizing plate.

The retardation plate and the polarizing plate function to polarizelight which enters from outside and is reflected by the metal electrode,and thus the polarization has an effect that the mirror of the metalelectrode cannot be viewed from exterior. Particularly, the mirror ofthe metal electrode can be blocked completely by forming the retardationplate with a quarter wavelength plate and adjusting an angle formed bythe polarization directions of the retardation plate and the polarizingplate to be π/4.

That is, the polarizing plate transmits only the linearly polarizedlight element among the external light entering the organic EL displayapparatus. In general, the linearly polarized light is changed intoelliptically polarized light by the retardation plate. When theretardation plate is a quarter wavelength plate and when the angle ofthe polarization directions of the polarizing plate and the retardationplate is π/4, the light is changed into circularly polarized light.

Generally, this circularly polarized light passes through thetransparent substrate, the transparent electrode, and the organic thinfilm. After being reflected by the metal electrode, the light passesagain through the organic thin film, the transparent electrode and thetransparent substrate, and turns into linearly polarized light at theretardation plate. Moreover, since the linearly polarized light crossesthe polarization direction of the polarizing plate at right angles, itcannot pass through the polarizing plate. As a result, the mirror of themetal electrode can be blocked completely.

Though a polarization light source and an image display apparatus usinga polarization component of the present invention have been describedabove, the present invention is not limited to the description. Thepolarization component of the present invention, using a reflectivepolarizer and a retardation layer that satisfy the requirements of thepresent invention, can exhibit effects of transmitting front directionlight only while reflecting and eliminating light in the obliquedirection. Furthermore, by adjusting a selective reflection wavelengthband of the reflective polarizer, the effects can be exhibited in a widewavelength range with less wavelength dependency. Moreover, sincedependency on the characteristics of a light source is decreased incomparison with parallelization and focusing system provided bycombination of an interference filter and a bright line emission lightsource as in a conventional technique, it can be used for various kindsof polarization light sources and image display apparatuses.

EXAMPLES

The following is a further description of the present invention, withreference to Examples and Comparative Examples. It should be noted thatthe present invention is not limited to these Examples alone.

(Instruments)

Equipment used in Examples and Comparative Examples will be describedbelow. Cold cathode ray tubes (CCFL) were products of ElevamCorporation. Backlights were products of Stanley Electric Co., Ltd. andof Tama Electric Co., Ltd. Light tables were products of HAKUBA.

The measurement apparatuses used were as follows.

(1) For measurement of a selective reflection wavelength band, aninstant multiple photometry system (trade name: MCPD 2000 produced byOtsuka Electronics Co., Ltd.) was used.(2) For haze measurement, a haze meter (trade name: HM 150 produced byMurakami Color Research Laboratory) was used.(3) For measurement of spectral characteristics intransmission/reflection, a spectrophotometer (trade name: U4100 producedby Hitachi, Ltd.) was used.(4) For measurement of characteristics of a polarizing plate, DOT 3(trade name) produced by Murakami Color Research Laboratory was used.(5) For measurement of retardation of a retardation plate or the like, abirefringence measuring apparatus (trade name: KOBRA 21D produced by OjiScientific Instruments) was used.(6) For measurement of brightness, a brightness photometer (trade name:BM7 produced by Topcon Corporation) was used.

Example 1

A polarization component including a reflective circular polarizer and anegative C-plate was produced as described below, and thecharacteristics were measured.

First, a reflective polarizer (reflective circular polarizer) includinga cholesteric liquid crystal layer was produced by using acommercially-available polymerizable nematic liquid crystal polymer(polymerizable mesogenic compound) and a chiral agent. The types andmixing ratios were determined so that the thus obtained cholestericliquid crystal layer would have a selective reflection wavelength bandwhose central value was 550 nm and whose width was about 60 nm.Specifically, LC 242 (trade name) produced by BASF was used for thepolymerizable mesogenic compound and LC 756 (trade name) produced byBASF was used for the polymerizable chiral agent, where the mixing ratiowas as follows.

Mesogenic compound:chiral agent=4.9:95.1 (weight ratio)

Specifically, the reflective circular polarizer is produced through thefollowing operations. First, a mixture of the polymerizable chiral agentand the polymerizable mesogenic compound was dissolved in cyclopentane,which was then adjusted to have a solute consistency of 20 wt %. To thissolution, 1 wt % of an initiating reagent (trade name: Irg 907 producedby Chiba-Geigy Co., Ltd.) was added so as to prepare a coating solution.

A PET film (trade name: Lumirror produced by Toray Industries) having athickness of 75 μn was prepared, whose surface was treated with arubbing cloth for alignment, thereby providing an alignment substrate.Next, on the treated surface of the alignment substrate, the coatingsolution was applied by using a wire bar. At this time, the amount ofthe solution was adjusted so that the thickness of the coating would be5 μm after being dried. This was then dried at 90° C. for 2 minutes, andfurther heated once to 130° C. as an isotropic transition temperature ofthe liquid crystal, then slowly cooled to retain a uniform alignmentstate. By curing this with ultraviolet irradiation at 80° C. (10mW/cm²×1 min.), a reflective polarizer A including a cholesteric liquidcrystal compound was obtained. A glass plate was prepared, on which atranslucent isocyanate-based adhesive (trade name: AD 249 produced byTokushiki Co., Ltd.) was coated to have a thickness of 5 μn, and thereflective polarizer A was transferred thereon, so that a desiredreflective circular polarizer was obtained. In a measurement, thisreflective circular polarizer had a selective reflection wavelength bandof 520 nm to 580 nm as had been designed.

Next, a negative C-plate including a polymeric liquid crystal compoundwas produced so that the central value of the cholesteric selectivereflection wavelength band would be 350 nm. Specifically, LC242 (tradename) produced by BASF and LC 756 (trade name) produced by BASF wereused for the polymerizable mesogenic compound and for the polymerizablechiral agent, where the mixing ratio was as follows.

Mesogenic compound:chiral agent=11.0:88.0 (weight ratio)

Specifically, the negative C-plate is produced by the followingoperations. First, the mixture of the polymerizable chiral agent and thepolymerizable mesogenic compound was dissolved in cyclopentane, whichwas adjusted to have a solute consistency of 30 wt %. To this solution,1 wt % of an initiating reagent (trade name: Irg 907 produced byCiba-Geigy Co., Ltd.) and 0.013 wt % of a surfactant (trade name:BYK-361 produced by BYK-Chemie Japan) were added.

A PET film (trade name: Lumirror produced by Toray Industries) having athickness of 75 μn was prepared, whose surface was treated with arubbing cloth for alignment, thereby providing an alignment substrate.Next, on the treated surface of the alignment substrate, the coatingsolution was applied by using a wire bar. At this time, the amount ofthe solution was adjusted so that the thickness of the coating would be6 μm after being dried. This was then dried at 90° C. for 2 minutes, andfurther heated once to 130° C. as an isotropic transition temperature ofthe liquid crystal, then slowly cooled to retain a uniform alignmentstate. By curing this with ultraviolet irradiation at 80° C. (10mW/cm²×1 min.), a laminate including the desired negative C-plateincluding the cholesteric liquid crystal and being formed on thealignment substrate was obtained.

In a measurement, the negative C-plate had retardation of 2 nm(substantially can be considered as 0) in the front direction (in-planeretardation) with respect to a light beam having a wavelength of 550 nm.The retardation was 160 nm (>λ/8) along a direction at 30° inclination.

The thus obtained reflective circular polarizer and the negative C-platewere used to produce a polarization component. First, the reflectivecircular polarizer including a reflective circular polarizer A formed ona glass plate was prepared. Next, the negative C-plate was transferredonto the reflective circular polarizer A. More specifically, atranslucent adhesive (trade name: AD249 produced by Tokushiki Co., Ltd.)was applied to have a thickness of 5 μm on the reflective circularpolarizer A, to which the negative C-plate formed on the alignmentsubstrate (PET film) was adhered. Then, the alignment substrate waspeeled off to leave the negative C-plate alone. On the negative C-plate,another reflective circular polarizer A was transferred further in thesame manner, thereby obtaining a desired polarization component. Thispolarization component includes, on a glass plate, a first reflectivecircular polarizer A, a negative C-plate and a second reflectivecircular polarizer A laminated in this order, and the respective layersare adhered via adhesive layers.

Next, performance of the obtained polarization component was evaluated.First, the polarization component was combined with a green diffusionlight source having a bright line spectrum in 544 nm so as to produce apolarization light source. Specifically, a G0 type cold cathode ray tubeproduced by Elevam Corporation and a light-scattering plate (haze: atleast 90%) were combined to provide a diffusion light source, with whichthe polarization component was further combined to provide apolarization light source, and this was disposed inside a direct-typebacklight apparatus. The light diffusion plate was disposed between thepolarization component and the cold cathode ray tube.

In an inspection of the characteristics of the polarization lightsource, light beams are emitted in the normal direction, but transmittedlight beams are decreased when the oblique angle is 30° or more, andsubstantially no light beams are emitted when the oblique angle isapproximately 45°. FIG. 16 shows a relationship between an emissionangle and a relative brightness for each case where the diffusion lightsource is used alone and where the polarization component of the presentexample was combined to provide a polarization light source.

FIG. 16 shows that the polarization component of the present example canfocus light beams efficiently in the front direction. This can beconsidered as a characteristic of the polarization component, sincegenerally it is considered as difficult to focus light beams in thefront direction by a lens or a prism in a direct-type backlight, unlikea case of sidelight-type backlight.

Next, the polarization component of the present example was disposed ona backlight (sidelight-wedge type backlight produced by Stanley ElectricCo., Ltd.) including three-wavelength cold cathode ray tube and used fora liquid crystal display apparatus, thereby the characteristics wereevaluated. Similarly in this case, light beams were emitted in thenormal direction, but transmitted light beams were decreased when theoblique angle was 30° or more. Since the polarization component did notcorrespond to the entire range of the visible light, light beams of blue(435 nm) and red (610 nm) were leaked due to incomplete stop-down ofangles, but a green spectrum (545 nm) with the highest visibility can beeliminated. As a result, operation as a focusing apparatus wasconfirmed.

Example 2

A polarization component was produced in the same manner as Example 1except for using a positive C-plate in place of the negative C-plate.First, a positive C-plate containing a polymeric liquid crystal compoundwas produced by using a liquid crystal monomer (referred to as apolymerizable nematic monomer A) represented by the following formula.

Specific operations for producing the positive C-plate are as follows.First, a polymeric nematic monomer A was dissolved in cyclopentane andadjusted to have a solute consistency of 30 wt %. Further, 1 wt % of aninitiating reagent (trade name: Irg907 produced by Ciba-Geigy Co., Ltd.)was added to this solution so as to provide a coating solution. A PETfilm (trade name: Lumirror, produced by Toray Industries, thickness: 75μm) was prepared, on which a light coating of a cyclohexane solution(0.1 wt %) of a release agent (octadecyl trimethoxysilane) was formed,which was then dried to form a perpendicularly-aligned film, thereby analignment substrate was formed. On the surface of this alignmentsubstrate on which the perpendicularly-aligned film was formed, theabove-mentioned coating solution was coated by using a wire bar. At thistime, the amount of the coating solution was adjusted so that thethickness of the film would be 2 μm after being dried. This was thendried for 2 minutes at 90° C., once heated to 130° C. as an isotropictransition point of the liquid crystal, then slowly cooled to retain auniform alignment state. Then, it was cured by irradiation ofultraviolet ray at 80° C. (10 mW/cm²×1 min.), thereby a laminate havinga desired positive C-plate formed on the alignment substrate was formed.In a measurement, the retardation of this positive C-plate with respectto a light beam having a wavelength of 550 nm was 0 nm in the frontdirection, and the retardation was about 170 nm (>λ/8) in a measurementwith an inclination of 30°.

A polarization component was obtained in the same manner as Example 1except that this positive C-plate was used in place of the negativeC-plate of Example 1. In an evaluation by using the thus obtainedpolarization component as in Example 1, the results for the performancewere substantially the same as those of Example 1.

Example 3

A polarization component including a reflective linear polarizer, aquarter wavelength plate and a C-plate was produced as described belowand its performance was evaluated.

First, a reflective linear polarizer was produced. That is, polyethylenenaphthalate (PEN) and naphthalene dicarboxylic acid-terephthalic acidcopolyester (co-PEN) were laminated alternately while adjusting thethickness of the thin films by a feed-block method, thereby obtaining amultilayer film of 20 layers. Then, this multilayer film was stretcheduniaxially. At this stretching, the temperature was about 140° C., andthe stretching ratio was about 3 in the TD direction. In the thusobtained stretch film, each thin film had a thickness of about 0.1 μm.Five laminate films of this kind of film including 20 layers werelaminated so that the product including 100 layers in total wasprovided, thereby a desired reflective linear polarizer (reflectivepolarizer B) was obtained. Due to the entire reflection power, thereflective polarizer B had a reflection function with respect tolinearly polarized light in a wavelength band not lower than 500 nm andnot higher than 600 nm.

Further, a polarization component was produced, using the reflectivepolarizer B. That is, a negative C-plate was produced as in Example 1,and to the both surfaces, a quarter wavelength plate (trade name: NRFfilm produced by Nitto Denko Corporation, having retardation (in-planeretardation) of 135 nm at 550 nm) made of a uniaxially stretched film ofpolycarbonate were adhered, and furthermore, the reflective polarizers Bwere adhered to the outermost surfaces so as to obtain a desiredpolarization component. The respective layers were bonded so that thein-plane slow axis direction of the quarter wavelength plate at theincidence side would be 45°, the C-plate had no axial direction, thein-plane slow axis direction of the quarter wavelength plate at theemission side would be −45°, and the transmission polarization axisdirection of the polarizer at the emission side would be 90°, when thetransmission polarization axis direction of the reflective polarizer Bat the incidence side was 0°. The respective layers were adhered byapplying an acrylic pressure-sensitive adhesive (No. 7 produced by NittoDenko Corporation) 25 μm in thickness, and the alignment substrate waspeeled off from the negative C-plate in order to use the liquidcrystal-containing layers alone. In an evaluation, the thus obtainedpolarization component used in substantially same manner as Example 1exhibited substantially the equal performance as Example 1.

Example 4

A polarization component including a reflective linear polarizer and ahalf wavelength plate was produced as described below, and itsperformance was evaluated. First, two reflective polarizers B producedin the same manner as Example 3, and a retardation film (a halfwavelength plate) obtained by biaxially stretching a polycarbonate film(produced by KANEKA Corporation) and having a front retardation of 270nm (measured wavelength: 550 nm) and having a Nz coefficient of 2.0 wereprepared. Then, the half wavelength plate was sandwiched by the tworeflective polarizers B so as to adhere these layers and obtain adesired polarization component. Regarding the bonding angles forrespective layers, when the transmission polarization axis direction ofthe reflective polarizer B at the incidence side was 0°, the in-planeslow axis direction of the half wavelength plate would be 45°, and thetransmission polarization axis direction of the polarizer at theemission side would be 90°. The respective layers were adhered byapplying an acrylic pressure-sensitive adhesive (No. 7 produced by NittoDenko Corporation) to have a thickness of 25 μm between respectivelayers. In an evaluation similar to that of Example 3, the polarizationcomponent was found to have performance substantially equal to that ofExample 3.

Example 5

A reflective circular polarizer (wide-band reflective circularpolarizer) having a selective reflection wavelength band in a widewavelength region was manufactured as described below. It was used witha C-plate in order to produce a polarization component, and itsperformance was evaluated.

First, a wide-band reflective circular polarizer was manufactured. Thatis, a nematic monomer A (as described above) and a chiral monomer Brepresented by the structural formulae below were prepared.

Next, the nematic monomer A and the chiral monomer B were mixed at apredetermined ratio and polymerized, which was used to produce acholesteric liquid crystal layer. Further, by changing the mixing ratioof the nematic monomer A to the chiral monomer B, four cholestericliquid crystal layers having varied selective reflection wavelengthbands were produced. In the production, EP Application 0834754 wasreferred to. The specific description is as follows.

Table 1 below shows a ratio for mixing the nematic monomer A and thechiral monomer B (weight ratio), and selective reflection wavelengthbands and the central wavelengths of the respective cholesteric layers,which are calculated therefrom.

TABLE 1 A/B Selective reflection wavelength band Central wavelength 9.2/1 430-490 nm 460 nm 10.7/1 480-550 nm 510 nm 12.8/1 540-620 nm 580nm 14.9/1 620-710 nm 660 nm

Next, the nematic monomer A and the chiral monomer B were polymerized tosynthesize a cholesteric liquid crystal compound. That is, each of themixtures of the composition as shown in Table 1 was dissolved intetrahydrofuran to prepare a 33 wt % solution, to which 0.5 wt % of aninitiating reagent (azobisisobutyronitrile) was added further. This wassubjected to a nitrogen-purge at 60° C., and then polymerized in a usualmanner. The thus obtained product was reprecipitate-separated withdiethyl ether and refined to obtain a desired cholesteric liquid crystalcompound.

A triacetylcellulose (TAC) film (trade name: TD-TAC produced by FujiPhoto Film Co., Ltd.) 80 μm in thickness was prepared, on which apolyimide layer having a thickness of about 0.1 μn was applied, and thepolyimide layer surface was rubbed with a rayon rubbing cloth so as toprovide an alignment substrate. Next, on the rubbed surface, a 10 wt %methylene chloride solution of the cholesteric liquid crystal compoundwas coated by using a wire bar so that the coated film would have athickness of 1.5 μm after being dried. This was heated at 140° C. for 15minutes, and subsequently kept and cooled at room temperature so as tofix the alignment state of the cholesteric liquid crystal compound,thereby obtaining a cholesteric liquid crystal layer. Theabove-mentioned operations were carried out for each of the thussynthesized cholesteric liquid crystal compounds so as to obtaincholesteric liquid crystal layers respectively having the selectivereflection wavelength bands as shown in Table 1.

The thus obtained four cholesteric liquid crystal layers were adhered ina certain order from the short wavelength side so as to obtain a liquidcrystal composite layer having a thickness of about 10 μm, therebyforming a desired wide-band reflective circular polarizer. The adhesionwas carried out by applying a transparent isocyanate-based adhesive(trade name: AD244 produced by Tokushiki Co., Ltd.) on a surface of eachliquid crystal layer, and subsequently peeling off the alignmentsubstrate (TAC film) of one side. In a measurement, the thus obtainedwide-band reflective circular polarizer had a selective reflectionfunction in a range of 430 nm to 710 nm.

A C-plate was produced in the same manner as Example 1, and thewide-band reflective circular polarizers were adhered on the bothsurfaces so as to obtain a desired polarization component. For theadhesion, a transparent pressure-sensitive adhesive (No. 7 produced byNitto Denko Corporation) 25 μm in thickness was applied between therespective layers, and the operations as in Example 1 were carried out.The upper and lower reflective circular polarizers were disposed so thatthe passing (reflected) circularly polarized light beams would rotate inthe same direction.

Next, performance of the polarization component of this Example wasevaluated as in Example 1. In an evaluation using a green diffusionlight source, a focusing performance similar to that of the polarizationcomponent of Example 1 was confirmed. In an evaluation using a backlightfor a liquid crystal display apparatus having a three-wavelength coldcathode ray tube, the focusing performance in this Example was as goodas that in Example 1. The polarization component of this Example wasexcellent in comparison with Example 1, in that the focusing performanceof the same level was exhibited in the entire visible light region.

Furthermore, the polarization component of this Example was disposed onanother backlight (a direct-type backlight using a cold cathode raytube, produced by Tama Electric Co., Ltd.), and the focusing performancewas evaluated. Similarly in this case, light beams were emitted in thenormal direction, but the transmitted light beams were decreased at anoblique angle of 30° or more. As a result, similar focusing performancewas exhibited in the entire visible light region.

Example 6

A polarization component was produced in the same manner as Example 5except that the thickness and retardation value of the C-plate waschanged, and the focusing performance was evaluated as in Example 5. Ina measurement of this Example, the C-plate had a thickness of 4 μm, andthe front retardation was 1 nm, the retardation at inclination of 30°was 100 nm (>λ/8).

FIG. 16 shows a relationship between an emission angle and relativebrightness of an emitted light beam for each case of using apolarization light source prepared by combining each of the polarizationcomponents in Examples 5 and 6 with a diffusion light source, and a caseof using the diffusion light source alone. This figure shows that thoughany of the polarization components exhibits excellent focusingperformance, the polarization component of Example 5 has a sharperfocusing angle and greater rise in the front brightness.

Example 7

The polarization component of Example 5 was assembled in a liquidcrystal display apparatus, and the display performance was evaluated.Specific description is as follows. First, for the liquid displayapparatus, a TFT liquid crystal display apparatus (diagonal: 11.3inches) taken from Dynabook SS3430 (trade name) produced by ToshibaCorporation was prepared. This apparatus uses a light source of asidelight type light-guide, and focus light beams to the front by meansof a prism sheet. Next, the prism sheet was removed from this liquidcrystal display apparatus, and onto a polarizer at the backside of theapparatus, a quarter wavelength plate (trade name: NRF-140 produced byNitto Denko Corporation) was adhered at an angle of 45° with respect tothe polarization axis, and furthermore, the polarization componentobtained in Example 5 was adhered to the surface. For the adhesion, atranslucent pressure-sensitive adhesive (No. 7 produced by Nitto DenkoCorporation) 25 μm in thickness was applied. In this manner, acommercially-available liquid crystal display apparatus was processed toobtain a desired liquid crystal display apparatus in which thepolarization component of Example 5 was assembled. In a comparison inperformance between the thus obtained liquid crystal display apparatuswith the polarization component and an unprocessed liquid crystaldisplay apparatus using a prism sheet, the property for focusing towardthe front was substantially equal to the display apparatus using a prismsheet, and the front brightness was improved by 20% after theprocessing.

This result demonstrates that the polarization component of the presentinvention is superior to conventional techniques such as a prism sheet.

Comparative Example 1

A polarization component was produced in the same manner as Example 1except that two reflective circular polarizer layers were bondeddirectly to each other without using a C-plate. In an evaluation for theperformance of this polarization component, the optical functions wererestricted to the same level as that of a single reflective circularpolarizer layer, while no phenomena such as improvement in selectivereflection rate and degradation in transmittance in an oblique directionwas observed.

Comparative Example 2

A polarization component was produced in the same manner as Example 1except that a quarter wavelength plate was used in place of the C-plate.The quarter wavelength plate used was an A-plate (trade name: NRF-140film produced by Nitto Denko Corporation, thickness: 50 μm) made of astretched polycarbonate film having a front retardation of λ/4 and Nzcoefficient=1.0. In an evaluation of the thus obtained polarizationcomponent, the front transmission was decreased by about ½ in comparisonwith Example 1. In addition, the transmittance in the oblique incidencedirection was not decreased, and the polarization component did notexhibit focusing and light parallelizing functions.

Comparative Example 3

A polarization component was obtained in the same manner as Example 3except that a commercially-available iodine-based absorptive dichroicpolarizer (trade name: NPF-EG1425DU produced by Nitto Denko Corporation)was used in place of the reflective polarizer B. In an evaluation ofthis polarization component, an effect of restricting visibility anglewas obtained due to the transmission characteristic in the frontdirection and absorption characteristic in the oblique direction, butthe front brightness was not improved due to the considerable absorptionloss.

(Brightness Evaluation by Use of a Light Table)

Each of the polarization components in Examples 1-6 and ComparativeExamples 1-3 was disposed on a commercially-available light table (aproduct of HAKUBA, three-wavelength fluorescent lamp, direct-typediffusion light source), and the brightness in the perpendicular rise(2° visibility angle) was measured by using a brightness photometer(trade name: BM7 produced by Topcon Corporation).

The measurement value was normalized by setting a value measured byusing a light table alone as 100. The measurement results are shown inTable 2.

TABLE 2 Relative brightness Example 1 80 Example 2 78 Example 3 72Example 4 70 Example 5 82 Example 6 90 Comparative Example 1 67Comparative Example 2 21 Comparative Example 3 39

As indicated in Table 2, the polarization components of Examplesexhibited excellent brightness enhancement effects in front directionseven when they were used for light tables. As shown in FIGS. 16 and 17,though the front relative brightness of the polarization component ineach Example exceeded 100 (the front brightness of the originalbacklight) in a use for a direct-type backlight used for a liquidcrystal display apparatus, the relative brightness was slightly lowerthan 100 in Table 2, since, for such a commercially-available lighttable, efficiency for a light beam reflected by a reflective polarizerto return in the normal direction is slightly inferior in comparison thecase where the direct-type backlight is used. Still, it is shown thatthe polarization component of each Example has a particularly excellenteffect in enhancing brightness in the front direction, as compared to apolarization component in each Comparative Example.

INDUSTRIAL APPLICABILITY

As mentioned above, a polarization component of the present inventioncan reflect an obliquely transmitted light beam toward a light sourceeffectively without degrading the transmission-polarization property ofa perpendicular incident light beam that contributes to frontbrightness. Furthermore, it is possible to further enhance thebrightness by converting an obliquely transmitted light beam reflectedtoward the light source (i.e., reflected polarized light) into a lightbeam that can contribute to enhancement of the front brightness. Also,by adjusting the selective reflection wavelength band of the reflectivepolarizer, the effects can be exhibited in a wide wavelength range withless wavelength dependency. Furthermore, since the polarizationcomponent of the present invention is less dependent on thecharacteristics of the light source, as compared to alight-parallelizing and focusing system or the like provided bycombination of an interference filter and a bright-line spectrum lightsource as in a conventional technique, it can be used for variouspolarization light sources and image display apparatuses. For example,when it is used as a polarizer at the backlight side of a liquid crystaldisplay component, a bright display that is excellent in visibility canbe obtained. Moreover, due to the excellent efficiency in using diffusedlight emitted from a light source, it can be used for forming ahigh-brightness polarization light source apparatus, and image displayapparatuses such as an organic EL display apparatus, PDP and CRT.

1. A polarization component comprising, at least, two reflectivepolarizer layers and an intermediate layer disposed between thereflective polarizer layers, wherein the two reflective polarizer layersare reflective linear polarizer layers that selectively transmit one oflinearly polarized light beams crossing each other at right angles whileselectively reflecting the other, the two reflective linear polarizerlayers have selective reflection wavelength bands for selectivereflection of polarized light, the bands overlapping each other at leastpartially, the intermediate layer comprises a single optical layer or alaminate of at least two optical layers, and the intermediate layer hasa function of transmitting an incident linearly polarized light beamwhile changing or not changing the polarization direction, according tothe incidence direction, the two reflective linear polarizer layers aredisposed at an angle so as to have in-plane slow axis directions fortransmitting a light beam that is included in incident linearlypolarized light and that enters in a direction (normal direction)perpendicular to the incidence surface while efficiently reflecting alight beam entering from an oblique direction.
 2. The polarizationcomponent according to claim 1, wherein the overlapping region of theselective reflection wavelength bands in the two reflective linearpolarizer layers comprises a wavelength range of 540 to 560 nm.